Automatic shifting power transmissions generally include a hydraulic control system, which establishes the drive ratios within the power transmission. The drive ratios are controlled by fluid-operated friction torque-transmitting mechanisms, such as clutches and brakes. These clutches and brakes are controlled in specific sequences to establish the plurality of drive ratios required by an automatic shifting power transmission.
The drive ratios are interchanged by engaging one or more torque-transmitting mechanism while disengaging one or more others. These ratio interchanges are controlled by shift valves, and therefore a transmission generally will require a minimum number of shift valves equal to the “log base two of the total number of forward speeds” with fractional values rounded up to the next whole integer (i.e., an eight-speed transmission would need a minimum of three shift valves).
With the increase in the number of ratios made available in a power transmission, the complexity and size of the housing or the valve body in which the hydraulic control structure is arranged is becoming increasingly large and requiring more real estate within the transmission, thereby increasing the overall size of the transmission.
Typical automatic transmissions use a single pump to provide both hydraulic pressure for the clutches and lubrication. This pump is forced to run at elevated pressures at all times, which creates losses. This invention uses a sealed “hydrostatic” system with three separate drivers to provide pressure to five different clutches. The advantage of this type of system is the ability to trap the clutch pressure by closing a valve and then shutting off the power (electrical or mechanical) to the pressure source. This eliminates the pump losses and improves total system efficiency.